The present invention relates to an improved ceramic dielectric composition comprising a chemically homogeneous distribution of a mixture of additives on the surface of ceramic powder particles, and, more particularly, to a method for enhancing the dielectric properties and sinterability of such compositions at low firing temperatures.
Ceramic dielectric formulations are used in the fabrication of a wide variety of microelectronic devices. The ceramic formulation must sinter to a dense hermetic body at a temperature below the melting point of a conductor co-fired with the ceramic in the multilayer structure, e.g., below about 1150.degree. C. for a 70% Ag/30% Pd alloy and below 1083.degree. C. for Cu. In the case of copper, the structure must also be fired in a non-oxidizing atmosphere to protect the conductors from oxidation during the sintering process. To achieve a dense hermetic structure when sintering at temperatures below 1150.degree. C., fluxes can be added to the ceramic dielectric formulations. U.S. Pat. No. 4,640,905, for example, describes a manganese doped zinc borate flux, and a low firing ceramic based on barium titanate with a high dielectric constant (K) for use in multilayer ceramic capacitors (MLC's) with Ag/Pd conductors. U.S. Pat. No. 4,845,062 describes a zinc borate frit used as a sintering aid for a magnesium titanate based ceramic in MLC's with copper conductors.
Traditionally, ceramic dielectric powders are prepared by physically blending a mixture of ceramic powders, with or without sintering aids. These ceramic powders can be processed rapidly and economically using readily available equipment to yield relatively dense monolithic bodies and multiphase ceramic bodies. The term "multiphase ceramic bodies" is used herein to refer to ceramic particles of one composition embedded in a matrix of a different composition. In MLC's, for example, a dielectric may contain discrete grains of essentially pure barium titanate in a shell of doped barium titanate or within a glassy matrix. This type of complex structure is difficult to synthesize by other methods. Typically, however, in the traditional blending method, the blend is inherently non-uniform because each component of the mixture has a different particle size distribution, particle morphology and surface properties. As a result, the fired ceramic is chemically non-homogeneous, contains pores and voids, but is tolerable since these deficiencies are smaller than the thickness of the ceramic layer and the scale of the printed circuitry.
Nevertheless, as electronic devices have become smaller and the miniaturization of electronic circuits has progressed, the traditional methods of producing ceramic powders have become inadequate, and the fabricated parts have become unreliable. The scale of the microcircuit elements has approached the scale of these defects.
Several approaches have been proposed to overcome these deficiencies. In one approach, the ceramic layer is created by direct decomposition of a solution of metal compounds, e.g., alkoxides. However, this approach can be slow, cumbersome and unreadily adaptable to mass production. Also, the reliable control of grain size and layer thickness, which are essential to modern electronic devices, can be difficult. U.S. Pat. No. 4,579,594, for example, describes preparation of an inorganic composite material by decomposing a solution containing at least two metals comprising a metal alkoxide oligomer, a metal chelate, a chelating agent and an aldehyde, which solubilizes the metal composition in an organic solvent. This method is inapplicable for coating fine ceramic particles due to binding of small particles and formation of large aggregates, which are unsuitable for production of a smooth ceramic. Moreover, multiphase ceramic bodies, preferred in many electronic applications, are not produced. U.S. Pat. No. 3,330,697 describes a method of preparing alkaline earth and lead titanates, niobates and zirconates by polymerizing the corresponding metal chelates with a polyhydroxy alcohol to yield a uniform distribution of dopants throughout the ceramic particle.
Another approach is described in International Publication Number WO 88/08830, that is, a technique in which differences in particle surface charge cause fine discrete particles of the dopant to associate with a larger ceramic particle. This approach is unsuitable for application of coatings which contain a soluble component. For example, boric oxides or the alkaline earth metal oxides may be desired components of the coating, but would tend to leach out when a filter cake is washed to remove by-product salts such as sodium chloride.
U.S. Pat. No. 3,490,927 describes an approach to bring about a chemical reaction between a desired additive and the surface of the ceramic particle, but is limited to situations where the appropriate reactivity exists between two chemical components. Such reactions tend to be slow and have limited commercial operation. The process involves coating the particle surface of a titanate powder by hydrolyzing a niobium or tantalum alkoxide of a high boiling polyhydroxy alcohol in the presence of a titanate powder.
The deficiencies of traditional blending methods have been overcome by the present invention. Also, the present invention can produce multiphase ceramic bodies which can be difficult to produce by other methods. More particularly, the present invention is an improved ceramic dielectric composition prepared by a process which allows for a distribution of a mixture of additives, including water soluble metal oxides, to be incorporated into a chemically homogeneous coating on the surface of ceramic powder particles.